It has become common in recent years to send emails to their destination with files attached. When the emails are sent, the attached files are encrypted at the transmission source depending on their confidentiality level. The transmission source sends, to the transmission destination, emails to which encrypted files are attached and separately notifies the transmission destination of the encryption information.
There are known technologies for encrypting emails to which files are attached. For example, there is a known technology for, if a file is attached to an email that is transmitted from the transmission source to the transmission destination, having the mail server automatically encrypt the attached file using a policy that conforms to the transmission destination. The transmission destination has already been notified of the encryption key and the file attached to the email is decoded at the transmission destination with this encryption key. With this technology, it is possible to reduce the time and effort needed to perform the encryption process and prevent leakage of information due to encrypting being forgotten.
Patent Document 1: Japanese Laid-open Patent Publication No. 2010-21746
Patent Document 2: Japanese Laid-open Patent Publication No. 11-232197
Patent Document 3: Japanese Laid-open Patent Publication No. 2002-330173
Patent Document 4: Japanese Laid-open Patent Publication No. 2009-145955
Patent Document 5: Japanese Laid-open Patent Publication No. 2010-21746
Patent Document 6: Japanese Laid-open Patent Publication No. 2004-362129
With the technology described above, the file to be attached to an email is encrypted by the mail server in accordance with the transmission destination of the email. Consequently, if there is a file that is not to be attached to an email and that is not to be sent to the transmission destination but that is erroneously attached to an email, the file is still encrypted in accordance with the transmission destination and sent. At this point, the file that is erroneously sent to the transmission destination can be decoded with the encryption key that has already been sent, as a notification, to the transmission destination.
As described above, with the technology in which the file attached to an email is encrypted by the mail server in accordance with the transmission destination, there is a problem in that a file that is erroneously attached to an email due to an error at the transmission source can be read at the transmission destination even if the transmission source does not want to permit it to be read. Furthermore, with the conventional technology, if an email with an attached file is sent, it is not possible to allow only a user who has the reading right to read the attached file.